Technical Field of the Disclosure
The present embodiment relates in general to impedance testing methods. More specifically, the present disclosure relates to an apparatus and method that increases the speed in which the impedance of a plurality of electrodes can be measured.
Description of the Related Art
Biopotential measurements like electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG) measure heart, muscle, and brain activity (respectively) over time by measuring electric potentials on the surface of living tissue. Nervous stimuli and muscle contractions can be detected by measuring the ionic current flow in a patient. This is accomplished by using a plurality of biopotential electrodes. The fidelity of these measurements is limited by the effectiveness of the connection of the electrode to the patient. The resistance of the electrode system to the flow of electric currents, known as the electric impedance, characterizes the effectiveness of the connection. Typically, the lower the impedance, the higher the accuracy of the measurements. To ensure good signal quality, skin under the electrodes should preferably be prepared by abrasion. This process can be time-consuming especially when multiple electrodes are used. Adding to that, skin preparation may damage skin, which is undesirable particularly in long-term applications, as the presence of electrodes and electrolytic gel can cause irritation and risk of infection. Electrodes may also come into contact with blood products when skin is abraded. As a result, a risk of an infection with a blood-born pathogen exists.
The quality of the electrode contact is quantified by its electrical impedance. Electrode contact impedance is a relevant factor while considering the signal quality. Electrode contact impedance is expected to have a relationship to electrode contact noise, which consists of thermal noise generated at the resistive elements of the contact, metal-electrolyte noise and electrolyte-skin noise. Thermal noise generated at the electrode contact is proportional to the resistive part of the electrode contact impedance. However, total noise generated at the electrode contact is generally significantly larger than expected thermal noise. Hence to get a better signal quality for biopotential measurements the electrode contact impedance must be minimized by minimizing the total noise generated at the electrode contact.
It is often desirable to measure electrode impedances continuously in real time while the patient is being monitored. Biopotential measurements allow a user to measure electrode impedance in order to verify proper electrode application. One method to measure impedance involves injecting a sine wave current (typically 20 Hz) into an electrode and measuring the resulting amplitude, then calculating impedance. This measurement process requires a certain amount of time to complete. As the number of electrodes in use continues to increase (128→256→512 channels), the time required to cycle through and measure all electrodes becomes very long (minutes).
A solution to this is to measure multiple electrodes in parallel. All measurement currents flow back to the device through the ground electrode. Depending on the ground electrode impedance, a common mode (CM) voltage is generated by the measurements currents. The common mode rejection of the amplifier removes this CM voltage. However, as the number of parallel channels (and the current) is increased, the CM voltage continues to increase and eventually becomes an error in the electrode impedance measurements.
One existing apparatus and method for determining electrode impedances of a bioelectric signal-monitoring/recording system includes an amplifier and electrodes connected between a subject and the amplifier. An example apparatus includes: a voltage source outputting a voltage signal; a switching arrangement including an input electrically connected with the voltage source for receiving the voltage signal; an output electrically connected with the amplifier and the electrodes; switches between the input and the output; and a controller for opening and closing the switches to establish signal paths between the voltage source and the output. The controller calculates the electrode impedances relative to voltage outputs of the amplifier for each signal path. However, this apparatus and method do not reduce the common mode voltage across the electrode and can result in an error in the measurement of impedance as the number of electrodes is increased.
Another existing system discloses a wireless system for brain monitoring/mapping of patients with neurological-disorders. It includes a plurality of electrodes each configured for surface abutment of brain tissue and main circuitry for placement outside the patient's body configured to transmit power at radio frequencies and send and receive data using infrared energy. This system includes a remote circuitry having multiple analog switches configured to enable each electrode to be selected to deliver a selected amount of electrical current to the selected electrode. An analog switch integrated circuit is configured to select an electrode for delivery of stimulation current. However, in this embodiment, the measurement of impedance is done by selecting a pair of electrodes, one selected electrode and a reference electrode, to complete the circuit in which the impedance measurement is performed. Hence this method tends to take a significant amount of time to complete. In addition, by measuring the electrical impedance of multiple pairs of selected electrodes with a reference electrode, errors can occur in the measurement of impedance.
Another existing system discloses an apparatus and method that determines the quality of the connection of an electrode to a patient. The apparatus includes at least three electrodes selectively connected to a patient for sensing an electrophysiological signal representing a patient parameter. A current source is connected to each of at least three electrodes, and the current source is able to apply both a positive current and a negative current. A control processor is connected to the current source and the electrodes. The control processor identifies a number of unique electrode pairs and controls the current source to simultaneously apply a positive current to one electrode and a negative current to another electrode of each identified electrode pair to determine the connection quality. However, in this method, impedance for each electrode is calculated using the current and voltage differentials for each electrode pair and a connection quality for each electrode is determined by comparing the impedance for each electrode to a threshold impedance which gives an inaccurate value.
Therefore, there is a need for a method and an apparatus for measuring impedance of a plurality of electrodes that would minimize the errors in impedance measurement providing an accurate result. Such a method of measuring impedance would reduce the common mode voltage and provide an efficient means of measuring impedance in a reduced amount of time. Such a method would allow measuring the impedance of a plurality of channels with accuracy. Such a method would not damage the skin by the presence of electrode and electrolytic gel thereby reducing the possibly of contracting a skin infection. Finally, such a method and apparatus measures the plurality of electrode impedance simultaneously with reduced measurement time.